Tissue imaging has proven to be a powerful tool for detection of cancers and other abnormalities within tissues. Mammography, in particular, is an excellent screening technology that has the ability to detect breast cancer several years earlier than physical examination. Early detection results in improved survival and an absolute decrease in mortality from the leading cause of non-preventable cancer deaths in women. The major problems encountered with mammography systems have to do with the difficulty of positioning and retaining breast tissue within the imaging space so that the tissue is thoroughly imaged and the images produced are interpretable.
One problem encountered with standard mammography systems is the difficulty of obtaining different perspective images of the breast tissue that can be effectively compared so that lesions can be identified. In order to detect cancers in breast tissues, mammographers must be able to distinguish malignancies from normal tissue. Breast tissue is composed of glandular tissue (parenchyma), fatty tissue, and supporting connective tissue. When imaged, normal parenchyma can look very much like cancer, and can often only be distinguished from cancer by comparing different perspective images of the breast. Malignancies can also be indicated by clusters of microcalcifications having a particular three-dimensional spatial arrangement that can often only be distinguished from normal clusters of microcalcifications by comparison of different perspective tissue images.
In particular, it is often necessary to compare standard perspectives of the tissue with "rolled" perspectives, in which the tissue is "sheared" (i.e. is deformed by forces tending to produce shearing strain) so that structures that were superimposed on each other in the standard (i.e."non-rolled") perspective are splayed or displaced relative to each other. An analysis of breast cancers missed on mammograms has revealed that the cancers that are most likely to be missed are those that are imaged with superimposed dense glandular tissue (Bird et al., Radiology 184:613-617, 1992).
One of the major problems encountered in mammography, therefore, is that tissue heterogeneity can mimic cancer configurations. There are problems associated with obtaining perspective images that can be easily compared so that malignancies can be identified. In standard mammography systems, "rolled" perspective images are achieved by direct manual manipulation of the breast tissue. A mammography system capable of shearing breast tissue without need for direct manual intervention is needed. A mammography system capable of shearing breast tissue that has already been positioned and imaged in a standard perspective, so that different perspective images of the tissue can be prepared without removing the tissue from the imaging space (and without removing compression, if compression has been applied, see below), would be particularly desirable.
Another problem encountered with standard mammography systems stems from incomplete capture of all relevant tissue within a tissue imaging space. Standard mammography procedures involve compression of the breast against an x-ray detector. The advantages of breast compression include: (i) improved retention of breast tissue for imaging; (ii) higher contrast images with better x-ray penetration and the ability to use lower energy x-rays; (iii) spreading of the internal structures of the breast so that images of the structures can be interpreted; (iv) reduced motion of the breast during imaging; (v) reduced x-ray dose required for imaging of thinner, compressed tissue; (vi) reduced scattering of radiation, resulting in higher contrast images; and (vii) proper positioning of the breast over the detector so that as much tissue as possible can be imaged at one time. In addition, compression forces the breast tissue closer to the detector, thereby improving the geometric sharpness of the image, and also permits uniform exposure over the majority of the breast tissue.
A significant problem encountered with standard compression systems is that compression can actually push tissue out of the field of view. This problem is particularly significant in mammography systems because breast cancers often develop close to the chest wall. Geometrically, the breast wraps around the chest and must be pulled away from the chest in order that x-ray shadows of the inner structures will be projected on the detector and will be recorded. If breast tissues are not held over the recorder, they will not be imaged and cancers will be missed. It is therefore desirable to retain as much tissue as possible within the imaging space, and particularly to be sure that tissue near the chest wall is pulled into, and retained within, the imaging space
Previous attempts to solve the problem of extrusion of tissue from mammography systems during compression have included attempts to pull breast tissue into the imaging space by suction. Suction, however, can cause discomfort to the patient. Furthermore, the reduced pressure associated with suction systems may rupture blood vessels and lead to severe bruising.
Other prior art systems designed to limit extrusion from mammography imaging spaces involve compression plates that either are oriented at acute angles (see U.S. Pat. No. 5,029,193 to Saffer), or have associated ridge members along the edge closest to the chest wall (see U.S. Pat. No. 4,962,515 to Kopans).
There remains a need for an improved mammography system capable of pulling breast tissue into the imaging space and/or of spreading or shearing tissue that is disposed in the imaging space.